Bragg reflectors are in common use in particular in the field of optical transmission. Such a reflector makes it possible to reflect a portion of the light that it receives.
More particularly, a Bragg reflector is defined by a periodic grating made of two materials having different refractive indices. The reflection ratio of such a device depends both on the difference between the refractive indices of the two component materials, and also on the geometry of the grating. It is proportional to the product .kappa..times.L, where L is the length of the grating and .kappa. is the "Bragg coefficient" which is related firstly to the difference between the refractive indices of the two materials, and secondly to the thickness of the grating.
A conventional reflector is shown in longitudinal section in FIG. 1A and in cross-section in FIG. 1B. For example, the reflector may be disposed facing a laser cavity so as to reflect a portion of the light wave emitted by the laser cavity to feed it back to said cavity and enable it to oscillate continuously.
The reflector is implemented on layers 2, 3 stacked by epitaxy on a substrate 1. In general, the substrate 1 is made of indium phosphide (InP) doped with n-type carriers. It may be covered with buffer layer serving to facilitate the epitaxial growth of the various layers to be stacked. The buffer layer is not shown in FIGS. 1A and 1B. The stacked layers perform different optical functions. In the example shown in FIGS. 1A and 1B, an active layer 2 also referred to as a "waveguide" layer is deposited on the substrate 1. The active layer 2 is buried in a "bottom cladding" layer 3 made of a III-V type material such as InP.
The Bragg grating is referenced 10 in FIG. 1A. It includes a material 4 based on InP, such as, for example, a quaternary material of the GaInAsP type, shown by hatching in FIGS. 1A and 1B, and an InP material 5 constituting a "top cladding" layer.
The grating is formed by growing a layer 4 of the quaternary material by epitaxy on the bottom cladding layer, by etching the layer 4 in a crenelated configuration, and then by filling in the resulting gaps with a top cladding layer of InP 5 doped with p-type carriers. Since the GaInAsP quaternary material 4 and the InP 5 have different refractive indices equal respectively to 3.3 and to 3, the resulting holographic grating 10 makes it possible to reflect part of the light wave that it receives.
For example, when a transmitter using wavelength division multiplexing (WDM) is to be implemented, the length L of the Bragg grating must be as short as possible to avoid too much reflection. The longer the grating, the higher the reflection. The mean reflection ratio of a Bragg grating should not exceed 30%.
In addition, a grating having a large refractive index step size offers the advantage of being usable for a large number of wavelengths because it enables reflection to be obtained over a wide spectrum window.